figure out the local type preference of the active candidate

mostly to find out if we're using turn/udp, turn/tcp or turn/tls. Will be useful for logging in #265.
This uses the same trick as the trickle-page to determine the local type preference, based on the comment in https://groups.google.com/d/msg/discuss-webrtc/NfCgW-8FhkE/3chOYjjPcboJ

It's probably in the wrong place and should be done in the completed event (which somehow I don't get?) but this should be enough to get the idea. Currently doesn't properly work for IPv6 (I think) since the local ip will have [] in the stats but not in the candidate line.

I'd still prefer getting the serverUrl in the candidate stats, but until then this hack allows gathering the data
worst hack ever, eh? :-)